RFC 7918

Transport Layer Security (TLS) False Start

Internet Engineering Task Force (IETF) A. Langley
Request for Comments: 7918 N. Modadugu
Category: Informational B. Moeller
ISSN: 2070-1721 Google
August 2016 Transport Layer Security (TLS) False Start
Abstract
This document specifies an optional behavior of Transport Layer
Security (TLS) client implementations, dubbed "False Start". It
affects only protocol timing, not on-the-wire protocol data, and can
be implemented unilaterally. A TLS False Start reduces handshake
latency to one round trip.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7918.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
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This document describes a technique that alleviates the latency
burden imposed by TLS: the client-side TLS False Start. If certain
conditions are met, the client can start to send application data
when the full handshake is only partially complete, namely, when the
client has sent its own ChangeCipherSpec and Finished messages (thus
having updated its TLS Record Protocol write state as negotiated in
the handshake) but has yet to receive the server's ChangeCipherSpec
and Finished messages. (Per Section 7.4.9 of [RFC5246], after a full
handshake, the client would have to delay sending application data
until it has received and validated the server's Finished message.)
Accordingly, the latency penalty for using TLS with HTTP can be kept
at one round-trip time.
Note that in practice, the TCP three-way handshake [RFC0793]
typically adds one round-trip time before the client can even send
the ClientHello. See [RFC7413] for a latency improvement at that
level.
When an earlier TLS session is resumed, TLS uses an abbreviated
handshake with only three protocol flights. For application
protocols in which the client sends data first, this abbreviated
handshake adds just one round-trip time to begin with, so there is no
need for a client-side False Start. However, if the server sends
application data first, the abbreviated handshake adds two round-trip
times, and this could be reduced to just one added round-trip time by
doing a server-side False Start. There is little need for this in
practice, so this document does not consider server-side False Starts
further.
Note also that TLS versions 1.3 [TLS13] and beyond are out of scope
for this document. False Start will not be needed with these newer
versions since protocol flows minimizing the number of round trips
have become a first-order design goal.
In a False Start, when the client sends application data before it
has received and verified the server's Finished message, there are
two possible outcomes:
o The handshake completes successfully: The handshake is
retroactively validated when both Finished messages have been
received and verified. This retroactively validates the
handshake. In this case, the transcript of protocol data carried
over the transport underlying TLS will look as usual, apart from
the different timing.

o The handshake fails: If a party does not receive the other side's
Finished message or if the Finished message's contents are not
correct, the handshake never gets validated. This means that an
attacker may have removed, changed, or injected handshake
messages. In this case, data has been sent over the underlying
transport that would not have been sent without the False Start.
The latter scenario makes it necessary to restrict when a False Start
is allowed, as described in this document. Section 3 considers basic
requirements for using False Start. Section 4 specifies the behavior
for clients, referring to important security considerations in
Section 5.
2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT","RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. False Start Compatibility
TLS False Start as described in detail in the subsequent sections, if
implemented, is an optional feature.
A TLS server implementation is defined to be "False Start compatible"
if it tolerates receiving TLS records on the transport connection
early, before the protocol has reached the state to process them.
For successful use of client-side False Start in a TLS connection,
the server has to be False Start compatible. Out-of-band knowledge
that the server is False Start compatible may be available, e.g., if
this is mandated by specific application profile standards. As
discussed in Appendix A, the requirement for False Start
compatibility generally does not pose a hindrance in practice.
4. Client-Side False Start
This section specifies a change to the behavior of TLS client
implementations in full TLS handshakes.
When the client has sent its ChangeCipherSpec and Finished messages,
its default behavior per [RFC5246] is to not send application data
until it has received the server's ChangeCipherSpec and Finished
messages, which completes the handshake. With the False Start
protocol modification, the client MAY send application data earlier
(under the new Cipher Spec) if each of the following conditions is
satisfied:
o The application layer has requested the TLS False Start option.

o The symmetric cipher defined by the cipher suite negotiated in
this handshake has been whitelisted for use with False Start
according to the Security Considerations in Section 5.1.
o The protocol version chosen by ServerHello.server_version has been
whitelisted for use with False Start according to the Security
Considerations in Section 5.2.
o The key exchange method defined by the cipher suite negotiated in
this handshake and, if applicable, its parameters have been
whitelisted for use with False Start according to the Security
Considerations in Section 5.3.
o In the case of a handshake with client authentication, the client
certificate type has been whitelisted for use with False Start
according to the Security Considerations in Section 5.3.
The rules for receiving data from the server remain unchanged.
Note that the TLS client cannot infer the presence of an
authenticated server until all handshake messages have been received.
With False Start, unlike with the default handshake behavior,
applications are able to send data before this point has been
reached: from an application point of view, being able to send data
does not imply that an authenticated peer is present. Accordingly,
it is recommended that TLS implementations allow the application
layer to query whether the handshake has completed.
5. Security Considerations
In a TLS handshake, the Finished messages serve to validate the
entire handshake. These messages are based on a hash of the
handshake so far processed by a Pseudorandom Function (PRF) keyed
with the new master secret (serving as a Message Authentication Code
(MAC)) and are also sent under the new Cipher Spec with its keyed
MAC, where the MAC key again is derived from the master secret. The
protocol design relies on the assumption that any server and/or
client authentication done during the handshake carries over to this.
While an attacker could, for example, have changed the cipher suite
list sent by the client to the server and thus influenced cipher
suite selection (presumably towards a less secure choice) or could
have made other modifications to handshake messages in transmission,
the attacker would not be able to round off the modified handshake
with a valid Finished message: every TLS cipher suite is presumed to
key the PRF appropriately to ensure unforgeability. Verifying the
Finished messages validates the handshake and confirms that the
handshake has not been tampered with; thus, secure encryption is
bootstrapped from secure authentication.

Using False Start interferes with this approach of bootstrapping
secure encryption from secure authentication, as application data may
have already been sent before Finished validation confirms that the
handshake has not been tampered with -- so there is generally no way
to be sure that communication with the expected peer is indeed taking
place during the False Start. Instead, the security goal is to
ensure that if anyone at all can decrypt the application data sent in
a False Start, it must be the legitimate peer. While an attacker
could be influencing the handshake (restricting cipher suite
selection, modifying key exchange messages, etc.), the attacker
should not be able to benefit from this. The TLS protocol already
relies on such a security property for authentication; with False
Start, the same is needed for encryption. This motivates the rules
put forth in the following subsections.
It is prudent for applications to be even more restrictive. If
heuristically a small list of cipher suites and a single protocol
version is found to be sufficient for the majority of TLS handshakes
in practice, it could make sense to forego False Start for any
handshake that does not match this expected pattern, even if there is
no concrete reason to assume a cryptographic weakness. Similarly, if
handshakes almost always use ephemeral Elliptic Curve Diffie-Hellman
(ECDH) over one of a few named curves, it could make sense to
disallow False Start with any other supported curve.
5.1. Symmetric Cipher
Clients MUST NOT use the False Start protocol modification in a
handshake unless the cipher suite uses a symmetric cipher that is
considered cryptographically strong.
Implementations may have their own classification of ciphers (and may
additionally allow the application layer to provide a
classification), but generally only symmetric ciphers with an
effective key length of 128 bits or more can be considered strong.
Also, various ciphers specified for use with TLS are known to have
cryptographic weaknesses regardless of key length (none of the
ciphers specified in [RFC4492] and [RFC5246] can be recommended for
use with False Start). The AES_128_GCM_SHA256 or AES_256_GCM_SHA384
ciphers specified in [RFC5288] and [RFC5289] can be considered
sufficiently strong for most uses. Implementations that support
additional cipher suites have to be careful to whitelist only
suitable symmetric ciphers; if in doubt, False Start should not be
used with a given symmetric cipher.

While an attacker can change handshake messages to force a downgrade
to a less secure symmetric cipher than otherwise would have been
chosen, this rule ensures that in such a downgrade attack, no
application data will be sent under an insecure symmetric cipher.
5.2. Protocol Version
Clients MUST NOT use the False Start protocol modification in a
handshake unless the protocol version chosen by
ServerHello.server_version has been whitelisted for this use.
Generally, to avoid potential protocol downgrade attacks,
implementations should whitelist only their latest (highest-valued)
supported TLS protocol version (and, if applicable, any earlier
protocol versions that they would use in fallback retries without
TLS_FALLBACK_SCSV [RFC7507]).
The details of nominally identical cipher suites can differ between
protocol versions, so this reinforces Section 5.1.
5.3. Key Exchange and Client Certificate Type
Clients MUST NOT use the False Start protocol modification in a
handshake unless the cipher suite uses a key exchange method that has
been whitelisted for this use. Also, clients MUST NOT use the False
Start protocol modification unless any parameters to the key exchange
methods (such as ServerDHParams or ServerECDHParams) have been
whitelisted for this use. Furthermore, when using client
authentication, clients MUST NOT use the False Start protocol
modification unless the client certificate type has been whitelisted
for this use.
Implementations may have their own whitelists of key exchange
methods, parameters, and client certificate types (and may
additionally allow the application layer to specify whitelists).
Generally, out of the options from [RFC5246] and [RFC4492], the
following whitelists are recommended:
o Key exchange methods: DHE_RSA, ECDHE_RSA, DHE_DSS, ECDHE_ECDSA
o Parameters: well-known DH groups (at least 3,072 bits), named
curves (at least 256 bits)
o Client certificate types: none
However, if an implementation that supports only key exchange methods
from [RFC5246] and [RFC4492] does not support any of the above key
exchange methods, all of its supported key exchange methods can be

whitelisted for False Start use. Care is required with any
additional key exchange methods, as these may not have similar
properties.
The recommended whitelists are such that if cryptographic algorithms
suitable for forward secrecy would possibly be negotiated, no False
Start will take place if the current handshake fails to provide
forward secrecy. (Forward secrecy can be achieved using ephemeral
Diffie-Hellman or ephemeral Elliptic Curve Diffie-Hellman; there is
no forward secrecy when using a key exchange method of RSA, RSA_PSK,
DH_DSS, DH_RSA, ECDH_ECDSA, or ECDH_RSA, or a client certificate type
of rsa_fixed_dh, dss_fixed_dh, rsa_fixed_ecdh, or ecdsa_fixed_ecdh.)
As usual, the benefits of forward secrecy may need to be balanced
against efficiency, and accordingly, even implementations that
support the above key exchange methods might whitelist further key
exchange methods and client certificate types.
Client certificate types rsa_sign, dss_sign, and ecdsa_sign do allow
forward security, but using False Start with any of these means
sending application data tied to the client's signature before the
server's authenticity (and thus the CertificateRequest message) has
been completely verified, so these too are not generally suitable for
the client certificate type whitelist.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492,
DOI 10.17487/RFC4492, May 2006,
<http://www.rfc-editor.org/info/rfc4492>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
DOI 10.17487/RFC5288, August 2008,
<http://www.rfc-editor.org/info/rfc5288>.

Appendix A. Implementation Notes
TLS False Start is a modification to the TLS protocol, and some
implementations that conform to [RFC5246] may have problems
interacting with implementations that use the False Start
modification. If the peer uses a False Start, application data
records may be received directly following the peer's Finished
message, before the TLS implementation has sent its own Finished
message. False Start compatibility as defined in Section 3 ensures
that these records with application data will simply remain buffered
for later processing.
A False Start compatible TLS implementation does not have to be aware
of the False Start concept and is certainly not expected to detect
whether a False Start handshake is currently taking place: thanks to
transport layer buffering, typical implementations will be False
Start compatible without having been designed for it.